1. Field of the Invention
This invention relates to an image forming method which utilizes an electrophotographic method or electrostatic printing method or the like applicable to copiers, printers, and fax machines and the like, an image forming apparatus, and a two-component developer applicable to image forming apparatus
2. Description of the Related Art
Generally, an electrophotographic method is recognized in the art as process utilizing a photoconductive substance. In the electrophotographic method, a latent electrostatic image is formed by various methods, on a latent electrostatic image carrier such as photoconductor. The latent electrostatic image is then developed with a toner, the toner is then transferred on to a paper or the like as necessary, fixed by heating or solvent vapor, to obtain copied images or printed images.
As means for developing latent electrostatic image formed on the latent electrostatic image carrier, wet developing processes and dry developing processes may be mentioned as examples.
The wet developing processes are processes in which a liquid developer is used. For dry developing processes, examples of methods include a method in which a toner containing colorant used as one-component developer dispersed in a binding resin, or a method in which a two-component developer containing a mixture of toner and a carrier.
Although each of these different means for developing a latent electrostatic image have various advantages and shortcomings, methods which utilize a two-component developer relatively have higher possibilities in responding to a higher-speed, longer-life requirements, compared to methods which utilize a single-component developer. Accordingly, dry developing processes utilizing the two-component developer are often employed presently and is widely used for primarily in medium- to high-speed copiers and printers.
Recently, a strong demand for higher definition and higher resolution in the copied or printed images has arose. In order to obtain such images having high definition and high resolution, in Japanese Patent Publication No. H6-82227/1994 (published), Japanese Patent Publication No. H7-60273/1995 (published), Japanese Patent Application Laid-Open No. H2-877/1990 (published), Japanese Patent Application Laid-Open No. H1-112253/1989 (published), Japanese Patent Application Laid-Open No. H2-284158/1990 (published), and Japanese Patent Application Laid-Open No. H7-295283/1995 (published), propose developers having small average particle diameter defined by the contained amount of the toner particles having particle diameter of 5 xcexcm or less, and the distribution thereof.
In these publications, it is disclosed that, toner particles having diameter of 5 xcexcm or less are mandatory components for forming high definition, high resolution images, and such toners containing small toner particles, when supplied smoothly during the development of latent images, contribute to generate faithful images of the latent images, that is, to generate images having outstanding reproducibility which do not protrude the contour of the latent images. On the other hand, there is a problem of an edge effect phenomenon in which the density around the center of an image becomes low compared to edge portions (i.e., corners) of the image. Such phenomenon appears more conspicuously as toner particles become smaller, for example 5 xcexcm or less. However, it also discloses that such phenomenon may be suppressed by defining the No. % of the toner particles having an intermediate particle diameter of 5 xcexcm or greater.
Moreover, as smaller the diameter of the particles become, it becomes more advantageous in terms of forming images of high resolution and high definition even if the content of toner particles having diameter of 5 xcexcm or less in an entire toner is 17 No. %, this accounts for no more than a mere 3 vol. % when expressed as a vol. % age. In such level of quantity, it is very difficult to selectively place toner of a small particle diameter of 5 xcexcm or less in the peripheral portions on a latent image. Furthermore, these toner particles are magnetic toner particles, containing 50 parts by mass or more of magnetic bodies relative to the binding resin. For that reason, magnetization in a magnetic field of 1 kiloersted (1 kxc3x6e=approximately 79.6 kAxc2x7m) becomes great, exceeding 20 emu/g. Accordingly, the toner becomes difficult to develop due to the magnetic bias effect, and, particularly in cases where toner having a toner particle diameter of 5 xcexcm or less is contained in the large quantity of 60 No. %, the toner becomes excessively charged (overcharged), and developing power further deteriorates. As a consequence, image density declines sharply, and this has constituted a problem. When the toner is difficult to develop, moreover, toner builds up on the carrier surface, a phenomenon called spent occurs, and the useful life of the developer is sharply reduced, which has also constituted a problem.
In order to prevent a spent phenomenon, conventionally, methods for coating the carrier surface with various resins have been proposed. However, although carriers coated in such a manner to exhibit outstanding charge characteristics, the threshold surface tension at the surface thereof is comparatively high, for such reason, the useful life thereof as a developer is not all that long, which has been a problem.
A carrier coated with an ethylene tetrafluoride copolymer is known. With this carrier, however, even though the problem of toner becoming spent does not readily occur due to the low surface tension, because the ethylene tetrafluoride copolymer is positioned on the most negative end in the frictional charge sequence, the carrier cannot be used when trying to electrify the toner to a negative polarity, which has been a problem.
Carriers have also been proposed as carriers of low surface tension which are coated with a coating layer containing a silicone resin. Examples include a carrier the surface whereof is coated with an unsaturated silicone resin and organo-silicone or silanol or the like mixed in a styrene-acrylic resin (U.S. Pat. No. 3,562,533, specification), a carrier the surface whereof is coated with a polyphenylene resin and an organo-silicone terpolymer resin (U.S. Pat. No. 3,847,127, specification), a carrier the surface whereof is coated with a styrene-acrylate-methacrylate resin and organo-silane, silanol, or siloxane or the like (U.S. Pat. No. 3,627,522, specification), a carrier the surface whereof is coated with a silicone resin (Japanese Patent Application Laid-Open No. S55-127567/1980 (published), and a carrier the surface whereof is coated with a resin-denatured silicone resin (Japanese Patent Application Laid-Open No. S55-157751/1980 (published). Thus, by using a carrier the surface whereof is coated with a silicone resin, resistance to becoming spent is improved, but, when the amount of toner having a particle diameter of 5 xcexcm or less is great, the recent demand for longer useful life cannot be satisfied, which has been a problem.
Art wherein a small quantity of toner particles having a diameter of 5 xcexcm or less is defined has been proposed, in Japanese Patent Application Laid-Open No. H4-124682/1992 (published) and Japanese Patent Application Laid-Open No. H10-91000/1998 (published), for single-component developing schemes. However, there is no provision concerning particle diameter distribution in a range wherein most of the toner particles that determine image quality exist, and the effectiveness of such art is limited to single-component developing schemes wherein single-component developers are used.
With the single-component developer schemes noted above, no use is made of a developer wherein carrier particles and toner particles are mixed, as in a two-component developer scheme wherein a two-component developer is used, but toner is held on a developing sleeve, either by electrical force produced by friction between the toner and the developing sleeve, or by magnetic force between a developing sleeve having a built-in magnet or magnets and toner containing a magnetic particle. Thereby, when an approach is made close to a latent electrostatic image, a drawing force in the direction of the latent electrostatic image for the toner particles produced by an electric field formed by the latent electrostatic image overcomes the bonding force between the toner particles and the developing sleeve, in which the toner particles are drawn to and adhere to the latent electrostatic image and the latent electrostatic image is made visible. Accordingly, with a single-component developer scheme, the advantage of being able to make the developing apparatus small is gained because there is no need to control toner density, but, because the number of toner particles in the development region is few compared to a two-component developer scheme, the amount of toner development toward the photoconductor is not sufficient, making it very difficult to cope with high-speed copiers, which has been a problem.
As a method of resolving such problems, a two-component developer scheme which requires no toner density control, such as described in Japanese Patent Publication No. H5-67233/1993 (published), has been devised. In this two-component developer scheme, developer in the vicinity of the developing sleeve takes toner into the developer in the toner supply portion, and toner is charged after controlling the developer with a layer thickness controlling member, thereby gaining the advantage of requiring no replenishing mechanism for replenishing toner or sensors for detecting toner density.
However, because the amount of developer cannot be made as great as in a conventional two-component developer scheme, in the case of a high-speed machine where the linear speed of the developing sleeve becomes high, the toner cannot be sufficiently charged, and the occurrence of ground fogging has been a problem. Also, when an attempt is made to impart sufficient charging to the toner, it is necessary to make the controlling stress at the layer thickness controlling member stronger, and due to the heat generated by the mutual collisions of the developer particles, a film of toner is formed on the carrier surface, the so-called spent phenomenon ensues, carrier charge characteristics decline as the time of use lengthens, and toner scattering and ground fogging and the like develop, which has been a problem.
Also, as noted earlier, for a developer used in a miniaturized developing apparatus, because it is necessary to impart charging to the toner supplied, in a short time, large amounts of fluidity enhancers have been added to the toner so that the toner supplied would mix quickly in the developer. When developers to which large amounts of fluidity enhancers have been added are repeatedly used, however, the excessive fluidity enhancer in the toner adheres strongly to the latent electrostatic image carrier, leading to the problem of abnormal images being produced with streaks in them. When the stirring stress on the developer is made large, moreover, in addition the spent phenomenon described earlier, the amount of charging in the toner becomes greater than necessary, resulting in the so-called charged-up phenomenon, which has also been a problem.
Furthermore, with the miniaturized developing apparatus noted above, the volume of developer is small, and the volume of toner held by the developer is small. Thereupon, when an original document having a lot of image area is repeatedly copied, toner consumption becomes excessive, and the toner concentration in the developer exhibits extreme changes, so that image density declines, which has also been a problem.
Furthermore, with the miniaturized developing apparatus noted above, the amount of toner uptake is different between places where developer action is vigorous and places where it is not vigorous, and between places where there is much developer and places where there is little. Thus the toner density becomes unstable in places, and image density irregularities or fogging readily develop, which has been a problem. Hoping to resolve such problems as this, in Japanese Patent Application Laid-Open No. S63-4282/1988 (published), art is disclosed for placing two toner supply members in the toner hopper, and causing developer to pass along the paths formed by those toner supply members, thereby resolving the problems of density irregularity and fogging in the long direction of the apparatus. With the art disclosed in that publication, however, because two toner supply members are used, the size of the developing unit becomes large and costs increase, which are problems. In this art, moreover, toner particle diameter and particle diameter distribution are critical. More specifically, when the numbers of particles measuring 5 xcexcm or less becomes great, toner fluidity deteriorates, and the problem of toner uptake becoming destabilized is encountered. When there are many large, coarse particles in the toner, moreover, the actual toner uptake amount decreases, and, particularly in cases where images that consume a large amount of toner are output, image density declines, and this too has been a problem.
Hoping to resolve such problems as these, many methods have been proposed which define the mean particle diameters, and the quantities to be added, of fluidity enhancers. In Japanese Patent Application Laid-Open No H2-43654/1990, for example, a method is proposed where fine silica powder having an average particle diameter of 0.05 xcexcm or less and titanium oxide particles having an average particle diameter of 0.1 xcexcm or less are added. However, although the addition of titanium oxide particles is effective in terms of environmental stability and image density stabilization, when a fluidity enhancer having an average particle diameter of 0.1 xcexcm or greater is used, the toner separates in the developing machine, and this has been a problem because it has resulted in ground fogging caused by toner fluidity deterioration and other factors detrimental to image quality.
The purpose of the present invention is to resolve the problems in the prior art described in the foregoing and to achieve the following objects. A first object of the present invention is to provide a two-component developer in which adequate charging is imparted to the toner and good images can be obtained which exhibit no toner scattering or texture smudging, an efficient image forming method which uses that two-component developer, and an image forming apparatus which carries that two-component developer and yields good images exhibiting no toner scattering or texture smudging.
A second object of the present invention is to provide a two-component developer in which good images can be obtained which excel in the reproducibility of fine lines and intermediate tones, an efficient image forming method which uses that two-component developer, and an image forming apparatus which carries that two-component developer and yields good images exhibiting no toner scattering or texture smudging.
A third object of the present invention is to provide a two-component developer which excels in environmental stability, and in stability over time when used for extended periods, an efficient image forming method which uses that two-component developer, and an image forming apparatus which carries that two-component developer and excels in environmental stability, and in stability over time when used for extended periods.
A fourth object of the present invention is to provide an image forming method and image forming apparatus small of size and inexpensive, which require no toner replenishing mechanism or toner density sensors, and a two-component developer to be carried in that image forming apparatus.